The difference is that the airline is selling you the right to sit on their airplane; the right to store your baggage on their airplane is another matter. In the case of the cell phone, your service contract is what gives you the right to use their network, and they are
selling you a phone to use on that network.
The special treatment is the DMCA, which forbids you from unlocking your phone regardless of the contract. This gives the cell companies an advantage by protecting a loss leader strategy: it is less risky for them to sell you a phone at a large discount which is locked, because it is illegal for you to unlock it whether or not you are under contract. It would be as if an airline sold you a discount ticket, and the law forbade you from having another company carry your luggage.